Microminiature vacuum tubes are being investigated for their potential of higher speed operation over solid state devices, occasioned by the fact that carriers in the tube-type devices travel in vacuum rather than through solid state semiconducting material. If the devices can be made sufficiently small to "miniaturize" the travel distances between the electron emitter (sometimes called the cathode) and the collector (sometimes called the anode), very high speed of operation is potentially available. That makes such devices very attractive for application such as very high speed switching devices at ultra-high frequencies.
Since such devices use no cathode heaters, potentials must be utilized which are adequate to cause the cold emission of electrons from the cathode for collection by the anode. The magnitude of the field voltages can be reduced if the anode and cathode are rather closely spaced, and if the cathode (or emitter) is shaped to provide a point or sharp edge which causes a concentration in field intensity at the point or line, enhancing the ability to emit electrons with a lower potential. One of the problems which has been encountered in such devices, however, is the reliable formation of emitter structures with the necessary sharply pointed characteristic. When such devices are formed in an array with multiple devices (multiple vacuum tubes) on the same substrate for interconnection and therefore integration, problems of reliability of the overall device can become even more acute when the processes do not assure that all of the emitters are properly formed, and therefore have the same characteristics.
As an example, FIG. 2 shows a form of microminiature vacuum tube disclosed in the proceedings of the International Electron Device Meeting of 1986 (IEDM '86) at page 776 and entitled "A Vacuum Field Effect Transistor Using Silicon Field Emitter Arrays". FIG. 2 shows a microminiature vacuum tube generally indicated at 10 based on a semiconductor substrate 1 such as n-type silicon. The upper surface of the substrate 1 is treated as by etching to produce a conical emitter 6. A layer 2 of insulating film such as silicon dioxide surrounds the emitter 6. Located on the film is a grid structure 3 and collector electrodes 4. Electrons emitted from the tip of the conical emitter 6 due to the electric field existing as a result of a potential applied from emitter to collector travel the arcuate path suggested by e.sup.- to be collected at the collector. A voltage applied to the grid 3 affects the field existing between the point of the emitter and the collector, and thus controls electron flow. The device can be used in a linear mode, or as a switch; in both cases, voltages applied to the grid control electron flow between emitter and collector.
It is important to note that the tip of the conical emitter 6 is shaped as it is to enhance the electrostatic field at the tip and thereby facilitate emission of electrons. If the tip were flatter, substantially higher potentials would be required to achieve the same magnitude of electron flow If the conical emitter 6 were shaped to be substantially shorter, higher potentials would also be required because of the increased distance between emitter and collector. Thus, the importance of the shape and disposition of the emitter are understood to be an important factor in achieving reliable and repeatable operation of vacuum tube devices such as illustrated in FIG. 2.
The process for forming the device of FIG. 2 is illustrated in FIGS. 3a-3c. As seen in FIG. 3a, the n-type silicon substrate 1 is patterned to produce a photoresist mask 5 defining the central area of the substrate in which the conical emitter is to be formed. A wet etching process is then carried out using an etching solution, such as a KOH aqueous solution. The substrate 1 underlying the photoresist 5 is underetched due to the isotropic nature of the wet etching process. As a result, when etching is completed, a sharp-edged configuration of conical shape is obtained, as illustrated in FIG. 3b.
When the process has proceeded to the stage illustrated in FIG. 3b, the photoresist 5 is removed and the cathode portion 6 is covered with a film of SiN, then annealed. Following that an SiO.sub.2 film 2 is formed over the remainder of the upper surface of the substrate 1 as shown in FIG. 3c. The SiN which had protected the emitter during the deposition of the SiO.sub.2 is then removed, and a grid structure 3 and collector structure 4 are deposited on the upper surface of the insulating film as indicated in FIG. 3c. Such electrode structures are deposited using conventional plating and lift-off techniques.
When the thus formed device, as better illustrated in FIG. 2, is disposed in a vacuum, and a DC potential is applied, biasing the cathode 6 negative with respect to the anode 4, an electric field is generated as suggested at e.sup.-. When that field becomes greater than 10.sup.7 V/cm, electrons are emitted from the cathode and collected by the anode. When the electrons reach the anode 4, an electric current flows, and when the device is used as a switch, is can be considered to be turned on. It is possible to control the electric field between cathode and anode by applying a voltage to the grid 3, thereby to control the switching operation. Because the electrons travel in a vacuum in such a device, their speed is greater as compared to the case where electrons travel in a semiconductor material. Such a device thus has the capability of even higher speed operation than solid state devices, and can provide a transistor which functions as a high speed switching element at ultra high frequencies.
The microminiature vacuum tube 10 illustrated in FIG. 2 by the production method described in connection with FIGS. 3a-3c relies on underetching the portion of the silicon substrate immediately underlying the photoresist in order to achieve the conical shape desired for the emitter. In wet etching, which is the process preferred for such underetching, when the degree of adhesion between the etching mask and the substrate is insufficient, it becomes difficult to control the sharpness of the conical tip with adequate reproducibility. Furthermore, because the etching rate is highly variable, and depends on the composition of the etching bath, the temperature of the liquid, the surface condition of the material to be etched, and other environmental conditions such as the degree of illumination of the device during etching, wet etching is not completely suitable for controlling tip sharpness of the conical emitter with adequate reproducibility. When the tip sharpness varies, the distribution of the electric field surrounding the tip varies, and this causes nonuniformity, from emitter to emitter, of the operating voltage needed to cause cold cathode emission. This should not be an overwhelming problem in the case where microminiature vacuum tubes are being manufactured in a laboratory for test, or in a small pilot operation, but when it is desired to produce such vacuum tube having high performance and repeatable and reliable characteristics in large commercial quantities, the problems will be substantially magnified.
FIG. 5 shows a further prior art structure which produces a finished product not substantially unlike the FIG. 2 embodiment in structure, but which is produced by a substantially different fabrication process. The fabrication technique is illustrated in FIGS. 6a-6d. There is shown a semiconductor substrate 1, preferably monocrystalline silicon, which is covered by an etchant mask which is then patterned as illustrated at 7 to expose a central conical aperture. It is noted that the aperture need not be completely conical, but that an elongate V-shaped structure is also appropriate in providing an emitter having a sharp discontinuity for enhancing electron emission. However, the conical form will be focused on herein. Having masked the device as illustrated in FIG. 6a, the conical aperture 8 is formed by wet etching, following which the mask 7 is removed. The device is then plated to cover the upper surface of the substrate and the walls of the conical aperture 8 with a metallic layer 6 which is intended to serve as the device cathode or emitter. The metal layer is typically thicker than a conventional conductive electrode, and is often formed of materials such as tungsten. Having covered the surface of the substrate 1 and the walls of the conical aperture 8 with a metallic layer 6 (as by sputtering or vacuum evaporation), operation switches to the rear surface of the substrate 1 to remove substrate material and expose the conical tip which is created by the metallic layer in the conical aperture. Thus, beginning with the partly completed device as illustrated in FIG. 6b, the substrate is thinned by etching from the rear surface until the tip 6a of the metal layer is exposed, as shown in FIG. 6c. Having thus exposed the conical tip, a silicon dioxide layer 2 is applied to the rear surface of the substrate, and gate electrode 3 and collector electrode 4 are deposited on the silicon dioxide layer as described in connection with the previous embodiment.
As shown in FIG. 5, device operation is like that of FIG. 2 in that when a biasing potential is applied between emitter 6 and collector 4, an electric field which concentrates at the tip 6a of the emitter is created as illustrated by the dashed lines e.sup.- to cause electron emission from the cathode and electron flow from cathode to anode. Although the conical emitter 6a is metallic as opposed to the thin film coated semiconductor of FIG. 2, the operation under the influence of an electrical field is similar.
The fabrication method illustrated in FIGS. 6a-6d also has substantial limitations with respect to uniformity, reliability and repeatability, although those issues are somewhat different than those associated with the FIG. 2 device and process. In the FIG. 6 process for fabricating the FIG. 5 device, since the metal structure which is to form the cathode (or emitter) is shaped inside a conical or V-shaped aperture which had previously been formed by wet etching, the tip sharpness of the electrode can be controlled with good reproducibility. While the depth of the recess 8 may vary with etching conditions, the tip sharpness does not substantially vary. It is therefore possible to obtain a relatively uniform electric field distribution surrounding the tip of the cathode, if the cathode is properly exposed by removal of the substrate material.
The removal of the substrate material, however, is not without its difficulties. As indicated in FIG. 7a, a rather substantial volume of substrate material must be removed in order to expose the conical tip 6a. The distance d2 identifies the bulk of the substrate which must be removed in order to expose the tip, and most typically the dimension d2 is in the range between about 100 and 500 microns. It is known that when rather massive substrate thinning is to be accomplished by etching, and the etching must proceed more than about 10 microns, what might have started as a planar surface prior to etching becomes relatively nonuniform by the time etching is completed. As a result, when a very substantial amount of material on the order of more than 100 microns is to be removed as suggested in FIG. 7a, and considering that only the tips 6a of the emitters are to be exposed, and the typical accuracy required must be of a micron or less, it will be appreciated that not all of the tips will be exposed to the same degree. FIG. 7b illustrates this condition in which a first tip 6A is substantially exposed as is the tip in FIG. 7a, whereas additional tips such as 6B remain buried in the substrate due to the uneven as-etched surface 1a. As a result, when a plurality of elements are produced at the same time, such as would be needed in an array of vacuum tube devices, the exposed portions and unexposed portions of the respective metal tips can coexist as demonstrated in FIG. 7b. The overall semiconductor part which results from a process as illustrated in FIG. 7b is not expected to be suitable for its intended purpose, and the yield of acceptable devices can be expected to be relatively low.